The primary marketing need for a lens shutter type camera has been good portability. In order to improve portability (we define it as lightweight and compactness), it is important to make shorten the total length of the picture-taking lens and to make miniaturize the diameter of the lens. It has recently become common to use a zoom lens in a picture-taking lens used for a lens shutter type camera. A zoom lens has a merit for a photographer to make it possible to take picture at will in accordance with variation of focal length. In particular, a camera including a high zoom ratio zoom lens having a zoom ratio of more than three is now typical.
As such a high zoom ratio lens, a zoom lens configured such that three or more lens groups are moved when the focal length is changed, has typically been used. In particular, a three-lens-group type having relatively small number of lens groups such as positive, positive, and negative (a lens type consisting of, in order from object side, a positive lens group, a positive lens group, and a negative lens group) has mainly been used.
There has been known following two focusing method for a lens shutter type camera, (1) and (2):
(1) A method for driving a focusing lens unit along the optical axis, wherein the focusing lens unit is holded by a shutter unit having a focus actuator which drives the focusing lens unit along the optical axis.
(2) A method for driving a focusing lens unit of a zoom lens along the optical axis, wherein the focusing lens unit is driven by a motor for both zooming and focusing along a trajectory of the focusing lens unit recorded on a cam by using a step type zoom lens stopping only at a plurality of predetermined lens positions. This method is disclosed, for example, in the Japanese Patent Publication No. 7-151952.
In the Japanese Patent Publication No. 7-151952, by using a two-group type zoom lens composed of a positive lens group and a negative lens group, a step type zoom lens stopping only at a plurality of predetermined lens positions is composed, which allows simplified construction by recording a trajectory of the focusing lens unit on a cam.
In other words, although the method (1) has a motor for zooming and a motor for focusing separately, the method (2) has only one motor used for both functions. Accordingly, the method (2) is preferable for compactness. However, it requires extremely high precision on the cam recording a trajectory of the focusing lens unit.
When the focusing according to the method (2) is performed in the Japanese Patent Publication No. 7-151952, a two-group type zoom lens composed of a positive lens group and a negative lens group is used. When a zoom ratio is to be enhanced, since stopping accuracy in the telephoto end state becomes extremely high, it is difficult to obtain required optical quality. Moreover, in a three-group type zoom lens, composed of a positive, a positive, and a negative lens group, used generally for a high zoom ratio zoom lens, it has been known that the method (2) is used for focusing near object. However, stopping accuracy required for obtaining predetermined optical quality cannot be realized because of complicated lens barrel structure.